On Jan 12, 2011, at 11:21 AM, David Skolnik wrote: > Room humidifiers and/or DamppChasers might be more effective, at > least in maintaining pitch. Room humidifiers are ineffective with an active HVAC system. The system changes the entire volume of air in the room much faster than any unit could replace it (a couple string faculty members here tried it, without any success whatsoever). There is no reason the same ductwork can't be used, depending how the system is designed. Typically there are enormous blowers moving two currents of air through two sets of ductwork, one for warmer, one for cooler air (and a separate exhaust system). Where there is temperature control (sometimes individual rooms, sometimes sets of rooms), the thermostat controls the mix of air coming from the two systems, by means of baffles entering a mixing box. If the air is humidified as needed at the source - down in the basement or up in the attic where the blowers are - it becomes relatively simple, as long as you aren't concerned with a very precise level (the problem being that the two temperatures of air will each have a different dew point/RH because of the difference in temp). You would control it for a dew point for the average target temperature (the two currents of air would both be adjusted to have the same dew point, or, to maintain a lower level of RH, to have the same minimum dew point). Refined humidity control is quite difficult, as it requires refined control of pressure throughout the building. It is the pressure differentials that cause flow of air into and out of rooms, from and to the ductwork. When doors are opened and closed, this changes everything. Windows make it virtually impossible, hence the move to have windows that can't be opened. This pressure system affects not only temperature control, but the effectiveness of the circulation and renewal of air. Circulation and renewal are the driving forces behind design, for human health reasons, so as not to get buildup of germs and toxins. No reason maintaining a minimum of humidity couldn't be added as another element of human health. These codes make it very expensive to have and run a public building, but they also protect the public health. Regards, Fred Sturm University of New Mexico fssturm at unm.edu
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